Metal organic chemical vapor deposition (MOCVD) is one of the standard methods of manufacturing thin (epitaxial) layered crystals. These layered structures are required in many semiconductor components including transistors and heterojunction lasers. Conventional MOCVD systems utilize gaseous reactants with at least one of the reactants being a metal organic compound. During vapor deposition the reactants are introduced into a quartz tube reactor vessel in which a substrate is held by a substrate holder and susceptor. The susceptor is used to heat the substrate so that the reactants pyrolyze and deposit out onto the heated substrate. Typically, the reactants flow past the heated substrate at one end of the reaction vessel and are exhausted at a distant end. Heating is required for the vapor deposition and crystal growth; since the walls of the reactor are relatively cool the reactants should only pyrolyze at the heated substrate.
Many high quality semiconductor components such as high electron mobility transistors and multi-quantum well laser diodes require extremely sharp heterojunctions between dissimilar layers of semiconductor crystal. When heterojunctions are not sufficiently abrupt, grading and other defects occur which decrease component performance, reliability and life. Creating abrupt heterojunctions is a difficult requirement to consistently fulfill because many sophisticated heterojunction structures require abruptly switching of material in ten to twenty angstroms thick layers.
In conventional MOCVD reactors, gases pass through a long entrance area to establish a stabilized flow and then pass over the heated substrate. The reaction gases substantially fill the reactor vessel before they are exhausted. A problem that arises with MOCVD reactors stems from the formation of gas eddies in the reactor. As a result of the gas eddies, small amounts of reaction gases can continue to flow over the substrate after gas injection has been terminated or the reactant gases have been changed. The tendency of residual amounts of reaction gases to be held in the reaction vessel affects the formation of heterojunctions. Residue gases result in the formation of a graded layer between dissimilar epitaxial layers. This grading results in a poor or inoperative semiconductor structure.
Inert gases have been used to disperse residue gases that can cause grading. After a first epitaxial layer is deposited on a substrate an inert gas is flowed through the reactor vessel prior to the deposition of a second epitaxial layer. Use of inert gases has not, however, been completely successful, since thermal degradation of prior deposited layers can occur while the inert gas is flushing the reactor vessel. The thermal degradation of exposed surfaces of epitaxial layers also results in degraded crystal structure.
Another frequent problem common to conventional MOCVD reactors results from the premature mixing of reaction gases. Prereaction of gases can lead to severe compositional and thickness non-uniformities in the growth of epitaxial layers. Indium phosphide and indium gallium arsenide systems are particularly affected by premature mixing of reaction gases.
Yet another problem that has arisen with conventional MOCVD reactors is a tendency towards non-uniform crystal growth due to variations in the reactant flow as it travels across the substrate. Conventional MOCVD reactor flow channels attempt to obtain a well behaved, non-turbulent reactant flow by having a long entrance length and/or quartz inserts to shape the flow traveling across the substrate. Unfortunately, reactant gases can become deleted as they pass over the substrate and deposit unevenly.
Finally, conventional MOCVD reactors utilize only a relatively small portion of the reactants by deposition on the substrate. Most of the reactants fill the reaction chamber and flow out through the exhaust without ever passing in close proximity to the substrate. This increases cost and aggrevates the problems related to disposing highly toxic gases.
In view of the above a need clearly exists for refining the MOCVD reactor system in order to produce higher quality, more uniform semiconductors.
It is also an object of the invention to provide an MOCVD reactor with an improved gas delivery system that prevents premature mixing of reactants.
It is a further object of this invention to provide an MOCVD reactor that offers substantially reduced grading at heterojunctions.
It is yet another object of this invention to provide a MOCVD reactor which substantially increases the proportion of th reactants that flow directly past the substrate and are deposited thereon.
It is yet another object of this invention to provide a MOCVD reactor with an inexpensive gas control system that satisfies the above objects.